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A neutrino detector

The neutrino detector at the University of Minnesota-operated Soudan Underground Laboratory.

Trails of tiny particles leave physicists beaming

University physicists unveil results from the world's leading neutrino experiment

By Deane Morrison

March 31, 2006

They are the greased pigs of the subatomic world, able to zip straight through the Earth without hitting anything. In fact, trillions whiz through your body every second, with neither party the wiser.

Called neutrinos (Italian for "little neutral ones"), these subatomic particles may hold the key to the origins of the neutrons, protons, and electrons that form the world we see. They have been under investigation for years, and the University of Minnesota is playing a key role in the most precise experiment to capture their essence.

On Thursday, March 30, University physicist David Petyt, speaking at the Department of Energy's Fermi National Accelerator Laboratory (Fermilab) near Chicago, announced the first results from the world's newest experiment to study the mass of neutrinos. The data give a peek into a world that exists on the edge of nothingness and paves the way for a detailed study of it.

The experiment wouldn't be possible without a massive detector at the University-operated Soudan Underground Laboratory. "Currently, the most significant particle physics experiment in the world is in Minnesota," says Peter Litchfield, U professor of physics.

To study the properties of these elusive particles, researchers from 32 institutions, including the University of Minnesota, last year began generating a beam of neutrinos at Fermilab and shooting them straight through the Earth to the 5,600-ton detector, located half a mile down in an old iron mine, where the layers of rock shield it from cosmic rays that would complicate the data. The neutrinos from Fermilab complete their 454-mile underground journey in just 25 thousandths of a second, and most just keep right on going. Only one in countless trillions of these neutrinos interacts with the detector and signals its presence.

Even with such a small mass, the abundance of neutrinos could have allowed them to play a role in forming the early universe and producing its asymmetric distribution of mass, Litchfield says. This asymmetry is fortunate for us; if the universe were perfectly symmetric, it would be a uniform mix of matter and energy, with none of the clumps we call galaxies, stars, and planets.

One more thing about neutrinos: They come in three types. If they change their identity en route to Minnesota, it indicates they possess mass. Such changes are called oscillation, and it can only be seen in neutrinos that travel long distances. (Particles that lack mass, as neutrinos were once suspected of being, cannot oscillate.) The experiment, called MINOS, involves 150 researchers from six countries. Petyt, a postdoctoral fellow, is one of the main data analysts for the project.

"After Fermilab, the University of Minnesota has the largest contingent of people in the project," says Litchfield, one of the MINOS collaborators. Other University physics professors on the project are Ken Heller, Marvin Marshak, Keith Ruddick, and Soudan lab director Earl Peterson, all of the Twin Cities campus, and Alex Habig of UMD. Mechanical engineering professor Tom Chase is also a collaborator. Neutrinos continually stream down from the sun and outer space. Both of these neutrino populations appear to oscillate, but it's difficult to say precisely how, because researchers can't tell how the neutrinos captured in a detector started out. But by generating their own beam of neutrinos, researchers can compare its composition at the source at Fermilab to the composition of the beam when it arrives in Minnesota. A previous Japanese experiment detected neutrino oscillation from lower-energy beams of human-generated neutrinos that traveled 150 miles, but MINOS has a more powerful beam and will measure things much more precisely.

After looking at their first six months of data, the researchers saw that the detectors in the Soudan lab had picked up fewer neutrino collisions than would have been expected if all the neutrinos coming from Fermilab had had an uneventful journey. The deficit implies that some neutrinos escaped detection by oscillating into other types during the trip.

"This is the currently most precise [human-generated] neutrino oscillation experiment, and it is as precise as the atmospheric measurements," says Ken Heller. "The experiment will continue for several more years, gathering data that will allow us to pin down the masses of the various types of neutrinos and how they oscillate from one to another."

The work goes to the heart of a basic question in physics: What is mass?

"If I asked students to define mass on an exam, the correct answer would be 'We don't know,'" says Heller. To probe the nature of mass, one has to look at the very smallest particles, at the boundaries between mass and masslessness. The mass of neutrinos is calculated to be no more than one ten-millionth the mass of an electron, adds Litchfield.

Even with such a small mass, the abundance of neutrinos could have allowed them to play a role in forming the early universe and producing its asymmetric distribution of mass, Litchfield says. This asymmetry is fortunate for us; if the universe were perfectly symmetric, it would be a uniform mix of matter and energy, with none of the clumps we call galaxies, stars, and planets.

In addition to working on the data analysis, University researchers were instrumental in designing and constructing the detector. For example, they worked with University undergraduate students to build about half the 80,000 particle detectors that pick up signals from neutrino hits.

"At Minnesota, MINOS has had the important participation of about 10 graduate students, six postdocs, 40 undergraduates, and five high school physics teachers," says Heller. "About 40 people were employed in construction of the detector. Also, [Rep. James] Oberstar has been very supportive of the project from the beginning."

The University and other collaborators have now proposed building a larger neutrino experiment to look at other aspects of neutrino oscillation, using a detector five times the size of the one in the Soudan lab. It would also be more sensitive, able to detect neutrinos from supernovas in the Milky Way or neighboring galaxies, Heller says.

Neutrinos are not the only major physics experiment to have found a home in the Soudan lab. From 1989 to 2001, the lab was home to an experiment that tested the ultimate stability of matter by searching for decaying protons. None were found, but the data were valuable to physicsts and helped set the stage for MINOS. Currently, the lab houses the Cold Dark Matter Search experiment of University physics professor Priscilla Cushman. This experiment is a hunt for components of the invisible "dark matter" that makes up about 24 percent of the mass of the universe.

To read John Updike's famous poem about neutrinos, click here.

For more information on MINOS, click here.